Protoporphyrinogen oxidase is an enzyme which convert protoporphyrinogen IX into protoporphyrin IX, in a tetrapyrrole synthesis system which exists universally in all organism.
In this synthesis system, heme is synthesized in case of a microorganism and an animal, on the other hand, chlorophyll in addition to heme is synthesized in case of a plant. This enzyme is thought to be a target enzyme of photobleaching herbicide, and the identification of the enzyme and the isolation of the gene has not been performed until now. HemG of E. coli has been isolated as a protoporphyrinogen oxidase gene in 1993 (Sasarman, A. et al (1993) Can. J. Microbiol. 39:1156-1161.), and HemY of B.subtilis has been isolated as a protoporphyrinogen oxidase gene in 1994 (Dailey, T. A. et al. (1994) J. Biol. Chem. 269:813-815.). In eukaryote, cloning of human protoporphyrinogen oxidase gene has been conducted in 1995 (Nishimura, K. et al. (1995) J. Biol. Chem.270:8076-8080.), and in the same year, cloning of mouse protoporphyrinogen oxidase gene has been conducted (Taketani, S. et al. (1995) Eur. J. Biochem. 230:760-765.). In plant, cloning of Arabidopsis thaliana and corn protoporphyrinogen oxidase gene has been conducted in 1995 (WO 96/34659).
Further, a gene of Arabidopsis thaliana . . . Heynh. and corn protoporphyrinogen oxidase tolerant to photobleaching herbicide has already been obtained by screening system using E. coli. 
The screening of a gene of Arabidopsis thaliana and corn protoporphyrinogen oxidase tolerant to photobleaching herbicide (WO 95/34659) has been done by using E. coli. Thus, it is not clear whether sufficient activity of such gene can be performed in a plant cell, and whether the tolerance is sufficient.